Fluid flow measurement in a fluid conducting pipe may be accomplished by various methods. One method includes inserting a pitot tube type of flow measuring device diametrically into the fluid carrying pipe, as shown and described in U.S. Pat. Nos. 4,154,100 and 5,773,726, to which reference is made for a more complete description. Briefly stated, in a measurement method using such apparatus, fluid pressure on the upstream and downstream sides of the pitot tube within the pipe are sensed by the pitot tube probe in order to ultimately produce a differential fluid pressure signal. Typically, flow rate is then determined from the sensed differential pressure using the equation Q=K.sqroot.DP, where Q is the flow rate, DP is differential pressure, and K is a constant empirically or theoretically determined for each combination of pitot tube and pipe diameter.
Following insertion into the fluid-conducting pipe the pitot tube is secured in the pipe by one of several methods. One such method is described in U.S. Pat. No. 4,727,159, where the distal end of the tube may be seated against the inside wall of the pipe opposite the wall through which the pitot tube is inserted and anchored. Another method is to secure the pitot tube to the pipe wall with a flange connection. The flange connection method does not allow for any adjustment of the insertion distance of the tube into the pipe and therefore does not insure contact between the tip of the pitot tube and the pipe wall. Such a mounting method does not allow for pipe expansion, as does the method described in the '159 patent. Because pipe dimensions are not held to a close tolerance, the length of the pitot tube designed for a "fixed" type of installation, such as the flange connection, must be dimensioned so as to accommodate a pipe that is less than the nominal diameter. Accordingly, in installations where the pipe has a diameter that is greater than the length of that portion of the pitot tube that is within the pipe, a gap will exist between the tip of the tube and the nearby inside wall of the fluid carrying pipe.
It has been discovered that the presence of a gap or space between the tip end of the pitot tube and the adjacent wall of the fluid conducting pipe can give rise to error in the flow measurement produced by the pitot tube. When such a gap is allowed to exist the fluid that flows under the pitot tube and through the gap, at a pressure of P.sub.static, migrates along the downstream side of the pitot tube toward the lower pressure fluid (P.sub.L) that exists there. This errant flow may adversely effect the K factor, producing an undesirable error in the calculated flow rate.
Accordingly, it is the primary object of the present invention to provide a pitot tube type of fluid flow measurement instrument with a fluid barrier mounted on its tip end that prevents fluid from flowing through the space between the distal end of the pitot tube and the adjacent pipe wall.
A second object of the invention is to provide a barrier to fluid flow through the gap between the end of a flow measuring probe inserted in a fluid-conducting pipe and the wall of the pipe that is adjacent thereto, where the barrier is substantially the same width and effective depth as the gap.
Another object of the invention is to provide a fluid flow blockage mechanism for the end of a pitot tube that can accommodate gaps having a variety of dimensions.
Other and further objects, features and advantages of the present invention will become apparent upon a reading of the following description of the invention, taken in conjunction with the accompanying drawings.